How to use 'pandas dataframe iterate over columns' in Python

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def __init__(self, df: DataFrame, columns: List[str]) -> None:
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    self._df = df
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    self._group_labels, self._group_position = self._create_groups(columns)
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    self._group_columns = columns
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    if len(self._group_position) == self._df.shape[0]:
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        warnings.warn("Each group contains exactly one row of data. "
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                      "Are you sure you are grouping correctly?")